An energy storage device, for example, an electric double-layer capacitor is a capacitor utilizing an electric double-layer phenomenon in which electric charges opposing each other are respectively accumulated on the surface of the polarizable electrode and the electrolyte interface when the polarizable electrode is put into contact with the electrolytic solution. This capacitor is typically constituted by a pair of facing polarizable electrodes, a separator electrically and physically separating the pair of polarizable electrodes, and an organic electrolytic solution. Activated carbon powder having a large charge storage interface, namely a large specific surface area is used as the polarizable electrodes.
Since this electric double-layer capacitor has a large electrode surface area and can achieve a significantly larger capacitance than an aluminum electrolytic capacitor that is recognized as having a large capacitance for capacitors, it has been mainly used for memory backup applications of household electric appliances. In recent years, large capacitance electric double-layer capacitors have been a focus of attention, and use thereof has been broadened to various applications, such as for OA equipment and industrial machinery, vehicles, and power generation by solar light and wind.
There are three kinds of this electric double-layer capacitor by structure: a coin type capacitor, a wound type capacitor, and a stacked type capacitor, and capacitance thereof is determined by the specific surface area of the electrode acting as the charge storage interface.
The coin type capacitor is formed by binding minute activated carbon fibers or activated carbon powder using a binder, impregnating an electrolytic solution into a separator that is interleaved between a pair of polarizable electrodes in parallel therewith, which have been made into a mat form and then punched out into circular forms, and then housed in a metal case which is also functioning as an exterior case, and caulking a metal lid via a gasket so as to seal it.
The wound type capacitor is formed by applying micronized activated carbon on a metal foil surface, which is a charge collector, using a binder in order to increase the electrode material surface area, thereby configuring electrodes, and then interleaving a separator between this pair of configured electrodes and winding them so as to make a capacitor element, which is then housed in a metal case, filled with an electrolytic solution, and then sealed.
The stacked type capacitor is formed by applying micronized activated carbon on a metal foil surface, which is a charge collector, using a binder in order to increase the electrode material surface area, thereby configuring electrodes, and then stacking these activated carbon electrodes alternately with a separator so as to make a capacitor element, which is then housed in either a metal case or a multilayer laminated film using thick aluminum foil, filled with an electrolytic solution, and then sealed.
Large capacitance electric double-layer capacitors for which applications have recently been expanding have been employing the wound type or the stacked type structure. The large capacitance type is used for regenerative energy for vehicles and also used for wind turbine generators and solar power generation systems having great load fluctuation. With this usage, excellent instantaneous charging and discharging and a long cycle life are desired.
Lowering the resistance of the capacitor is necessary for making the electric double-layer capacitor have a long cycle life. If the internal resistance is high, heat generates from internal loss due to the resistance when charging and discharging a large current over a short period of time, thereby degrading performance as a result of the generated heat. Improvement of various members such as electrode material and electrolytic solution is actively carried out for lowering the resistance of the electric double-layer capacitor, and demand for lower resistance of separators has grown.
Moreover, in order to improve productivity in the wake of the expansion of the market due to expansion of applications of the electric double-layer capacitor, a separator that is strong enough to allow improvement in productivity without adversely affecting the internal resistance and leakage current characteristics of the separator is in demand.
A regenerated cellulose fiber separator described in Patent Document 1, a multilayer separator containing regenerated cellulose and synthetic fibers described in Patent Document 2, and a double-layer separator resulting from stacking a layer containing regenerated cellulose fibers and a porous polyolefin film layer described in Patent Document 3 are the examples of suitable separators for the conventional wound and stacked type electric double-layer capacitors.
Certain patent documents as described herein are as follows:
Patent Document 1: JP 2000-3834A
Patent Document 2: JP 2013-171905A
Patent Document 3: JP 2011-35373A
Using a separator made of 100% by mass refinable, regenerated cellulose fibers as described in Patent Document 1 has a problem that the separator breaks in the manufacturing process of the wound and stacked type electric double-layer capacitors due to low tear strength.
This is thought to be due to the following reason. A sheet of paper made using minute fibrils of several tens of nm to several μm that are obtained by highly refining the refinable, regenerated cellulose fibers is a sheet having excellent ESR characteristics formed with very dense fiber mats but not increasing density since the rigidity of the fibrils is high and therefore do not easily collapse. Moreover, since inter-fiber hydrogen bonding increases due to increase in fibrils, the value of tensile strength can be increased.
However, while the tensile strength improves as a result of increasing inter-fiber bonding by refining refinable, regenerated cellulose fibers, the tear strength will rapidly decrease if the fibers are further refined to increase the inter-fiber bonding. That is, since the tensile strength and the tear strength due to the inter-fiber bonding of the fibers that have been refined to a certain degree have a reciprocal relationship, the more the tensile strength improves, the higher the fibers are refined and the more the tear strength decreases.
Here, there is a problem that the leakage current of the electric double-layer capacitor increases since not only the tensile strength but the shielding property decreases if refining is restricted so as to improve the tear strength.
Furthermore, a separator having excellent mechanical strength using a multilayer separator including high a density layer resulting from mixing refinable, regenerated cellulose and synthetic fiber, and a low density layer resulting from mixing refinable, regenerated cellulose and synthetic fiber is proposed as in Patent Document 2. However, the separator used in the working example of Patent Document 2 has problems that the inter-fiber hydrogen bonding is weakened into a separator with a large amount of fuzz due to a high content of synthetic fiber, thereby some of its fibers falling off due to the fuzz of the separator made when fabricating an electric double-layer capacitor, resulting in an increase in the leakage current of the electric double-layer capacitor.
This is because the shape and strength of the cellulose separator are maintained not only by mechanical strength due to intertwining of fibers but also by the chemical force of the hydrogen bonding among cellulose molecules.
Furthermore, a separator having excellent mechanical strength using a double-layer separator that results from stacking a porous polyolefin layer on a mixed layer of refinable, regenerated cellulose and synthetic fiber is proposed as described in Patent Document 3. However, there is a problem that the porous polyolefin film has fewer air gaps than the cellulose separator, thereby inhibiting ionic conduction in an electrolytic solution, resulting in degradation of ESR.